1. Field of the Invention
The present invention relates to a varnish and a prepreg, in particular, to a halogen-free varnish with composite main resin having two kinds of phosphorus-containing novolac epoxy mixed with a bisphenol—A novolac epoxy resin, a prepreg made from the varnish, and a substrate made from prepregs.
2. Description of Related Art
The printed circuit board (PCB) is widely used in many applications and fields, for example, electronic cards are inserted on PCBs. Thus, PCBs are known as a base of the electronic industry. Therefore, study and research on quality of PCBs is in order to improve the performance of electronic devices.
A PCB is made by pressing the prepregs, and copper clad laminate (CCL), or copper film together. The prepregs are made by dipping glass fabrics into varnish and then curing the resin on the fabrics to form thin prepregs.
Regarding environmental protection, halogen-free PCB's are generally becoming more and more developed for electronic applications. Phosphorus-containing compounds are usually added into the halogen-free PCB for enhancing the anti-flammability, or fire-retardancy of the PCB. Traditionally, the varnish can contain either primary resin with a phosphorus-containing compound “pcc” or curing agent with phosphorus-containing compound (pcc). Alternatively, a phosphorus-containing-compound may be added into the varnish for improving the anti-flammability of the PCB.
For example, the traditional varnish contains a single phosphorus-containing compound known as HCA (9,10-dihydro-a-oxa-10-phosphenanthrene-10-oxide) for improving the anti-flammability of the prepregs/PCBs. However, the toughness properties of the PCBs made by the traditional resin with a single phosphorus-containing compound cannot meet the requirements for electronic products.
Consequently, relating to the resolution of improvable defects illustrated hereinbefore, the inventors of the present invention have designed and now offer a well-designed solution for effectively eliminating such defects.